Stabilized Lasers Contribute to Geology

To the casual observer, most of the Earth appears to be quite stable. Only catastrophic events suggest a planet history of more subtle but persistent change.

Recently, the development of very sensitive instruments for detecting crustal deformation has shed light on the dynamics of an active, living system that exists just beneath our feet.

The Earth's crust has tides, like oceanic tides, that cause the ground to rise and fall about a foot twice a day. However, stable strain measurements cannot easily be taken without a strain meter.

The
long-base laser strain meter near the Salton Sea in California provides
baseline high-precision strain data for studies of the seismic cycle in
Southern California, and for comparison with crustal deformation
measurements.
Frank Wyatt, a research geophysicist at Scripps Institution of Oceanography at the University of California in San Diego, started working on strain meters in the late 1960s, when lasers were rarely used. His first working model came online in 1968 to measure very small deformations of the ground.

In those early years, Wyatt and his colleagues used devices such as quartz rods and taut wires that strain under crustal deformation. Now employing a helium- neon laser, he has accurately recorded what is happening at the ground surface.

Wyatt described his strain meter as a Michelson interferometer comprising a 15-cm-diameter vacuum pipe that spans 700 m. The pipe isolates the laser beam from the atmosphere so that variations in temperature and pressure won't affect measurements.

A beamsplitter sends part of the beam into one arm of the strain meter to a reflector that is only a fraction of a meter away. The other part of the beam travels through the vacuum pipe to a distant reflector. Both reflectors return light that rejoins at the beamsplitter on its way to a detector.

If the beams are out of phase, the resulting beam destructively interferes at the detector, creating bands of darkness, or fringes. By counting fringes, researchers can determine changes in distance.

Global positioning systems can also measure changes in distance, with an accuracy of 1 mm. This is useful for long-term deformation measurements. Wyatt said his strain meter can measure movement to within 1 µm.

This high spatial resolution requires a very stable laser. Wyatt uses a polarization-stabilized laser referenced to an iodine-stabilized one. He said he chose his primary laser from Micro-g Solutions Inc. because it offers the best frequency stabilization.

Tim Niebauer, Micro-g's president, said the laser's active feedback -- developed at the National Institute of Standards and Technology -- is the key to its stability. "A lot of lasers are modulated: The frequency dithers back and forth. Ours doesn't dither. It's always at one frequency. For interferometry, that's good."